A known power seat apparatus for a vehicle includes a speed reduction device that reduces a rotational speed of an output shaft of a drive source such as an electric motor and then transmits the resulting rotational speed (i.e., the rotation) to a drive shaft driving a movable portion of the power seat. U.S. Pat. No. 6,390,264B2 (hereinafter referred to as Reference 1) discloses such speed reduction device including a casing constituted by two members, i.e., a holding member (a casing body) and a cover member (casing cover). The holding member holds a worm gear and a helical gear that perform a speed reduction operation in a rotatable manner. The cover member covers the holding member. Then, four engagement projections are formed at an outer circumference of the cover member while engagement portions are formed at the holding member so as to face the respective engagement portions. A worm gear that rotates in the same direction and at the same speed as those of the output shaft of the motor (i.e., the motor output shaft) is assembled on an end portion of the holding member. A helical gear having an axial center perpendicular to an axial center of the worm gear and meshed with the worm gear is assembled on the holding member. The cover member is attached to the holding member in such a manner that the engagement projections of the cover member engage with the respective engagement portions of the holding member, the engagement portions then being riveted to thereby fix the cover member to the holding member. Accordingly, the casing of the speed reduction device is constituted by the two members in order to simplify an assembly of the worm gear and the helical gear. Besides the riveting, a fixing method of the two members may be generally achieved by a tightening with multiple small screws. In addition, in a case of mounting the speed reduction device itself to a seat frame, for example, fitting screws penetrate through three engagement holes formed at an outer peripheral portion of the holding member so that the holding member is meshed with the seat frame as disclosed in Reference 1.
However, according to the speed reduction device disclosed in Reference 1, the attachment of the cover member to the holding member and the assembly of the speed reduction device itself on the seat frame are separately and independently performed from each other. Thus, exclusive components and processes are necessary for attaching the cover member to the holding member and for assembling the speed reduction device itself on the seat frame, thereby increasing the number of components and processing. In addition, because the helical gear receives a large radial load and a thrust load from the worm gear, the casing is required to have a high bearing rigidity for holding both the worm gear and the helical gear so that a stable mesh between the gears is ensured. As a result, the holding member and the cover member after being attached to constitute the casing are both required to have a high holding force. Further, a general requirement for quickly and precisely assemble the holding member and the cover member on each other is still present.
A need thus exists for a speed reduction device for a power seat which is not susceptible to the drawback mentioned above.